It is well known that in an automobile the electrical outlet intended for use with a cigarette lighter is a convenient electrical connection point from which to draw power for various accessories such as a citizens' band (CB) radio, for example. Connectors designed specifically for this purpose are known from the Spring 1988 Radio Shack Catalog No. 419, model numbers 270-1560 and 270-1562 shown on page 147.
When an accessory to be powered is connected to the cigarette lighter receptacle and turned on, it draws power from the automobile's battery and charging system. A problem arises when the automobile engine is turned off because the charging system is thereby disabled and the accessory, if left turned on, begins to discharge the battery. If this condition persists, the battery will be discharged to the point where its ability to be used in starting the automobile will be impaired or precluded.
A circuit for monitoring battery voltage and shutting off a regulated power supply connected to it when the battery discharges to a predetermined low level of voltage is known from an article in Electronic Designer's Casebook, 14-A, p. 80 published by McGraw-Hill, Inc., entitled "Battery discharge triggers alarm and shuts off supply" by Dean Jeutter. It is noted that because this circuit allows discharge of the battery to a predetermined low level, the battery may not be in a condition to be used in starting the automobile.